You can install Ubuntu using the option to install its boot record to the partition instead of installing to MBR (Master Boot Record - only one for the whole disk !)
(it will give a warning which can be ignored).
Later a boot entry for Ubuntu in Windows BCD (Boot Config Data) store can be added to boot Ubuntu.
Windows boot manager will be responsible for booting Windows 7 and Ubuntu.
Ubuntu can be installed on any partition type - primary or logical - and it is good to have some 20 GB of unpartitioned space for Ubuntu (installation will create two partitions -> root and swap).
To add a boot entry for Ubuntu in Windows BCD store:
1. Create boot sector loader.
2. Copy the file /boot/grub/boot.img from Ubuntu over to Windows \boot folder.
3. Amend boot sector loader to point to boot.img.
To access files in Ubuntu from Windows - download and install "ext2fsd" - version 0.50 works great for me.
If you need help on creating a boot sector loader - http://boyans.*****.com
and read how to.